1. Field of the Invention
The present invention relates to an apparatus for measuring electromagnetic characteristics, such as magnetization at cryogenic temperatures, electrical resistance in a magnetic field, Hall effect and so forth. The invention particularly relates to an apparatus for measuring electromagnetic characteristics using a helium 3 refrigerator that is able to generate cryogenic temperatures down to 0.3 K and measure electromagnetic characteristics at the cryogenic temperatures, by utilizing a conventional MPMS with helium 4 that is able to generate a cryogenic temperature in the order of 1.8 K and then using helium 3 and the addition of a simple apparatus.
2. Description of the Prior Art
Magnetization measurement is an indispensable means of research into the physical properties of various materials, such as magnetic substances and superconductors. With respect to such magnetization measurement, currently the measurement apparatus called the MPMS (Magnetic Property Measurement System), made by Quantum Design Inc., of the United States, is widely used, being the de facto standard in measurement systems.
With the inclusion of a high-temperature option the MPMS is able to measure temperatures up to 800 K, but the lower limit is only 1.8 K, achieved using reduced-pressure liquid helium 4. For this reason, in making researches into specific superconductors, such as Ru or Re superconducting oxides having a superconducting transition temperature of around 1 K and various heavy-electron superconductors, that have recently attracted keen attention, the MPMS cannot be used to perform magnetization measurement. Since pNPNN that is a pure organic ferromagnetic substance developed for the first time in the world has a ferromagnetic transition temperature of 0.6 K, researches have been made using a handmade apparatus.
Since a temperature only in the order of 1.8 K can be achieved with an MPMS widely employed in the prior art, as described above, it has been unable to perform magnetization measurement in relation to the aforementioned various materials that have attracted keen attention. For magnetization measurement below 1.8 K, an apparatus has to be specially fabricated. This makes it a special-order item, which is very costly. In addition to this, its lack of general utility makes it difficult to use the manufactured apparatus in a wide range of fields.
Moreover, it is very difficult to configure such a special-order apparatus so that it can measure direct-current magnetization using a superconducting quantum interference device (SQUID), so instead it is often used to measure alternating-current magnetization. However, the low sensitivity of measurement based on alternating-current magnetization makes it impossible to directly determine the magnitude of magnetization. Furthermore, even when direct-current magnetization is measured, conventional measuring methods are used, such as the extraction method, vibrating sample method and magnetic field sweep method; measurement using a SQUID of ultrahigh sensitivity has not been possible. For these reasons, the benefits of ultrahigh sensitivity provided by using a SQUID have not been obtained.
Moreover, as the name indicates, an MPMS is essentially an apparatus for measuring magnetization at cryogenic temperatures and cannot readily be used to measure electrical resistance or Hall effect.
Thus, a main object of the present invention is to provide an apparatus for measuring cryogenic electromagnetic characteristics that is capable of high-sensitivity measurement of magnetization characteristics at a cryogenic temperature in the order of 0.3 K generated using a widely employed MPMS (Magnetic Property Measurement System).
Another object of this invention is to provide an apparatus for measuring cryogenic electromagnetic characteristics that can use the above-described MPMS to measure electrical resistance and Hall effect as well as magnetization at cryogenic temperatures.
The basic idea of the present invention is to use an auxiliary rotary pump to decrease to 1.5 K a 1.8 K space provided by an MPMS, insert therein a small, vertically-movable helium 3 refrigerator, and move the whole refrigerator vertically within a pickup coil to measure direct-current magnetization. The helium 3 refrigerator can reduce the temperature of a sample to 0.3 K. Unlike a case in which just the sample is moved up and down in liquid helium 3, moving the whole refrigerator up and down enables the sample to be maintained at a constant temperature. Based on this basic technical concept, the present invention employs the configuration described below to resolve the above problems.
That is, the apparatus for measuring electromagnetic characteristics according to this invention comprises a sample rod with a sample fixed to a lower part thereof; a helium 3 refrigerator, in which the sample rod is inserted, having a main pipe that forms around the sample rod a space that is cooled by helium 3; means for supplying helium 3 to the helium 3 refrigerator; an inner tube portion, into which the main pipe is inserted, that supports at an upper part thereof the refrigerator; an outer tube that cools an outer periphery of the inner tube with helium; a liquid helium container that supplies liquid helium to the outer tube; and means for measuring electromagnetic characteristics of the sample.
To supply helium 3 to the main pipe, the means of supplying helium 3 to the helium 3 refrigerator can be connected to the main pipe by a bellows that maintains an interior thereof in an airtight state.
The main pipe can be equipped with a vacuum insulation member for insulation from a surrounding space.
The outer tube can have main pump and auxiliary pump piping.
The apparatus for measuring electromagnetic characteristics according to the present invention can further comprise a control unit connected to the helium 3 refrigerator and the sample rod for performing magnetic field control, sample temperature control and measurement activities.
The means for measuring electromagnetic characteristics of the sample can be constituted by a pickup coil that generates an induced electromotive force that is proportional to magnetization of the sample, and a SQUID that detects the generated electromotive force, to measure the sample""s magnetization.
The means for measuring electromagnetic characteristics of the sample can be constituted by a constant-current power supply that provides a flow of DC or AC measurement current between current terminals of the sample and a voltmeter for detecting differences in electrical potential between voltage terminals of the sample, to measure electrical resistance and Hall effect of the sample.
As described in the above, by utilizing an MPMS that is widely employed in the prior art, the apparatus for measuring electromagnetic characteristics according to the present invention can form a sample space inside the main pipe having a temperature as low as 0.3 K. In addition to using this to measure the magnetization of a sample, it can also be used to measure the electrical resistance and the Hall effect of the sample.
Further features of the invention, its nature and various advantages will be more apparent from the accompanying drawings and following detailed description of the invention.